Japanese Patent Laid-Open Publication No. 2003-139069 discloses a conventional air-conditioning system including a refrigeration cycle, which employs an electric compressor to compress a refrigerant. This electric compressor is provided in a housing body with a compression portion, a motor for driving the compression portion, and a motor drive circuit for driving the motor. In the motor drive circuit, circuit components (heat generative components) are positioned where the components can exchange heat with an intake refrigerant entering the compression portion.
When the circuit components reach a predetermined temperature or a higher temperature, the rotational speed of the motor is increased to a predetermined rotational speed to thereby increase the flow of the intake refrigerant. The refrigerant cools the circuit components, allowing the cooling operation to be maintained without stopping the motor.
However, the increase in rotational speed of the motor may cause the circuit components to generate heat by themselves, resulting in insufficient cooling even during the increased flow rate of the intake refrigerant. This leads to another predetermined temperature, which is higher than the aforementioned predetermined temperature. In this case, the motor is to be stopped and eventually the cooling operation has to be stopped.